The present invention relates to a time division electronic switching system in which the interfaces of a signal processor with a push-button dial signal receiver and a multi-frequency signal receiver are simplified and expansion of the system is facilitated.
In a conventional time division electronic switching system, the main structural elements of the time division electronic switching system are a central processing unit, a signal processor and a time division switch including a speech path memory. The time division electronic switching system is connected to the subscribers and to other telephone offices through line circuits and trunk circuits, respectively. Furthermore, the time division electronic switching system includes a tone trunk for dial tones and call progress tones, a signal receiver circuit for receiving dial signals, especially push-button dial signals, and a multi-frequency signal receiver circuit for converting into analog signals the signals transmitted to other telephone offices and for converting to digital signals analog signals received from other telephone offices. The signal processor comprises, as main elements, a signal distributing circuit, a switch control circuit and a scanning circuit. The scanning circuit of the signal processor periodically makes a status scan of the line circuits and trunk circuits looking for the on-hook or off-hook state of a subscriber or the state of the trunk circuit. In the speech path memory, speech representation signals are transmitted while writing into and reading out of the speech path memory not only speech signals of the subscribers but also speech signals from other telephone offices. With respect to the site of the speech path memory and storage for the on-hook/off-hook information of the line circuit, one bit is sufficient per circuit, but 4 bits or 5 bits per line circuit are necessary for storage of the push-button dial signal from the receiver circuit and multi-frequency signal receiver. Furthermore, if the analog-digital conversion system is a Pulse Coded Modulation (PCM) system, 8 bits per circuit are necessary for the speech path memory. Accordingly, in the conventional electronic switching system, interfacing the time division switch, including the speech path memory or the signal processor, with another unit is complicated. Therefore, the conventional electronic switching system is disadvantageous in that the possible physical equipment locations are limited because of the system structure and expansion of the system is difficult.